Modern digital communications circuits are complex devices that contain, and are connected to, many active components. Testing them is difficult. This difficulty is compounded by the fact that a typical communications line is only maintained and operated by the service provider up to the point at which it enters the customer premises. Beyond that point of entry, termed the demarc or demarcation point, the line and the equipment attached to it is the responsibility of the customer.
When a fault occurs on a line, the service provider and the customer try to determine the location and cause of the fault by each testing the line on their side of the demarcation point. However, even the simplest digital line such as a T1 line, is a complex system, as described in detail by, for instance, U.S. Pat. No. 5,173,896 to Dariano entitled “T-carrier network simulator”, the contents of which are hereby incorporated by reference.
A typical issue that arises in attempting to test such T1 lines is that neither the service provider nor the customer are always entirely sure that they are actually testing to the demarcation point. The demarcation point is usually a physical Network Interface Unit, sometimes referred to as a “Smart Jack” because it is capable of responding to commands to set itself into a loop back mode. However, other components that may be attached to the line may also setup loop-backs at other points. It is also common for physical loop backs to be setup for local testing or to avoid alarm signals reaching other parts of the system. In a complex network a tester may not be aware that these loop backs are in place and may erroneously think they have tested the circuit to the demarcation point when in reality they have only tested it as far as some physically set loop-back put in place because of some other problem or test program.
Another common situation is that different spans of a T1 may be incompatibly configured. The portion of the T1 line and associated equipment on the enterprise or customer side may, for instance be configured for D4 framing, while the service provider T1 and equipment may be configured for ESF framing. In such a circumstance, each party tests and finds their span to be operational, but the complete line does not work.
That situation is often compounded by each group only knowing their own test equipment, and using their own jargon to describe their testing procedures. Often neither side understands or is convinced by the other sides test procedures.
All too often these types of issue give rise to a situation where both the service provider and the customer claims to have tested the communications line on their side of the demarcation point and found no fault, but the line is still not operational. Resolving such standoffs can be a costly and time wasting exercise. In practice, resolution of such standoffs is often only achieved when technicians from both the service provider and the customer meet at the demarcation point and jointly test the line. They do this in order to be sure that the line is really being tested from the demarcation point in each direction. Moreover, such a meeting allows them to test the line with the same equipment and to each see the same results, thus helping resolve any issues of whether the tests are appropriate, the results conclusive or the configurations different. However, such meetings can be difficult to schedule and costly, both in terms of the technicians' time and in the downtime of the line.
What is needed is a simple way to diagnose communication line faults and incompatibilities that allows service provider and customer technicians to test communications lines in a way that they are all convinced that the test reaches to the demarcation point, with an agreed upon set of diagnostic tools, in a way that provides them all with the same data and understanding of that data, without the need for a costly physical meeting.